1. Field of the Invention
The present invention relates to an antenna coil having such a structure that the resonant frequency thereof can be adjusted, which is used, for example, in an RFID (Radio Frequency Identification) communication device (reader/writer) that operates in the HF (High Frequency) band.
2. Description of the Related Art
According to a related art for loop antennas used for such RFID communication devices, a plurality of trimming lines for adjusting inductance, forming shortcut connections for an innermost loop of an antenna coil, is provided, and the resonant frequency is adjusted by cutting the trimming lines one by one as appropriate.
According to the related art described above, particularly, by forming a branched line extending with a considerable length from one side of the loop toward the inner side of the loop, and arranging a large number of trimming lines in parallel at substantially rectangular intervals in the lengthwise direction of the branch line, ladder-like trimming lines projecting considerably toward the inner side of the loop are formed. Furthermore, according to the related art, it is possible to form the branch line at a slant angle with respect to one side of the loop so that the trimming lines have a trapezoidal structure, whereby the amount of change in line length (the amount of change in inductance) that occurs by cutting each of the trimming lines becomes constant. Therefore, according to the related art, it is possible to adjust the resonant frequency of the antenna coil step by step by cutting the trimming lines one by one.
According to the related art, however, since a large number of trimming lines projects considerably toward the inner side of the loop, even after cutting of the trimming lines, as well as before cutting of the trimming lines, the basic shape of the loop is distorted. Thus, the shape of distribution of magnetic fields generated from the antenna during communication is distorted by the trimming lines and the branch line, so that communication performance is degraded. Particularly, if a region where the magnetic flux density is higher than or equal to a certain level is considerably deviated from that in the basic shape, a sufficient magnetic field is not generated for an antenna at the other end of communication, so that communication might fail.
Furthermore, although the antenna structure according to the related art is suitable for adjusting the resonant frequency step by step (at a regular pitch) by cutting trimming lines one by one, in order to adjust the resonant frequency by a large amount, it is necessary to cut a large number of trimming lines, which makes the adjustment task laborious. Particularly, with the structure according to the related art, it is not possible to adjust the resonant frequency by first roughly adjusting the resonant frequency and then changing the inductance in small steps to adjust the resonant frequency to a target value. Thus, the antenna coil according to the related art is not suitable for the task of adjusting the resonant frequency using various methods of adjustment.